CRETACEOUS FORMATIONS. 3 



ing lower upon the fore than upon the hind half of the centrum, and giving, in the 

 pair, a contour somewhat like that of a saddle ; I do not, however, insist upon this 

 as a constant character of the cervicals of this species. In the present vertebra 

 one of the venous orifices is larger than the other ; but in a second, of similar size 

 and contiguous position, they show the usual equality. The flatness of the ter- 

 minal surfaces is remarkable, and betokens restriction of the movements of the 

 neck of the species. On the similarly flattened under surface the venous foramina 

 (fig. 4, v) open nearer the anterior than the posterior border. 



In a cervical vertebra, of similar size and proportions, the neurapophysial sur- 

 faces are more concave in the longitudinal direction. As the cervical series 

 approach the back the centrums increase in length, while preserving about the 

 same relative breadth. In the vertebra, figs. 5 — 7, the costal surface (pf)has risen to 

 the neurapophysial one {np), with which it has become confluent ; the inferior tract of 

 the centrum now describes a convexity between the two costal surfaces, though it 

 is but slight ; the contour of the terminal surface accordingly presents the form of 

 a transversely elongate ellipse (fig. 6). The fore-and-aft contour of the under 

 surface is very slightly concave, almost flat. The posterior border of the costal 

 surface is produced, forming the beginning of a parapophysis (fig. 7). The 

 neurapophysial surfaces are slightly excavated, with a defined but hardly raised 

 border ; they are undulated, smooth, with scattered foramina ; their breadth is now 

 one third more than their length. In the posterior cervical (fig. 7) the venous 

 canals on the neural surface show the same inequality as in fig. 3. 



In the vertebra in which the costal surface has wholly passed upon the neur- 

 apophysis (fig. 10, np), and which, from the proportions of length to breadth, is to 

 be reckoned as coming from the beginning of the dorsal series, the sides of the 

 centrum are excavated under the neurapophysial surfaces ; but below the excava- 

 tion, which is not deep, the longitudinal contour is as nearly straight as in the 

 antecedent vertebra. 



To one of the terminal surfaces of this vertebra (fig. 11) adheres the remnant 

 of the lower valve of the spondyloid shell — Dianchora striata .- the living Spondylus 

 Gussoni, which most resembles the characteristic Green-sand bivalve, dwells at 

 great depths in coral-beds of the Mediterranean. We may conceive, by analogy, 

 that the carcass of the dead Plesiosaur, sinking and decomposing in a similar 

 chalky manufactory, left its scattered bones to serve as the resting-places of those 

 bivalves of its locality and period which, like the modern smooth and spiny oysters, 

 anchor themselves for life after a brief locomotive period. 



Towards the middle region of the back the centrums gain in vertical diameter, 

 and somewhat in length, with a diminution of their transverse diameter. The 

 concavity of the non-articular surface from before backward is still greatest near 

 the neurapophysis, but has less the aspect of a circumscribed depression than in 



